Personal displays are growing in importance with increasing proliferation of mobile and wearable communication, information and entertainment devices, in addition to their more traditional applications for simulators, mixed reality and other head-mounted applications. In many situations a binocular personal display system is preferred to a monocular personal display system for its superior convenience of use over protracted periods. Nevertheless, a binocular personal display requires stringent alignment of the lines-of-sight of the two displayed images. Relatively small misalignment can lead to serious user discomfort, headaches, nausea, and, in extreme cases, symptoms of sea-sickness and other ailments. In general, binocular alignment, requires careful design of the binocular device and often involves alignment procedures. The line-of-sight of the two eyes must be adjusted to lie in the same horizontal plane, bisecting both eyes through their centers, to an accuracy of a few minutes of an arc. Within this horizontal plane the lines-of-sight should be adjusted to cross at the region of the apparent distance of the observed object (this horizontal offset between the lines-of-sight is termed parallax). Both of these angular alignments must be maintained for different inter-pupil distance (IPD) of the user. The IPD of the population typically varies between 54 and 75 mm. A mechanical adjustment for the distance between the pupils is required to ensure a correct IPD, such as is well-known in common Porro Prism Binoculars. Suitable mechanisms for aligning and maintaining the binocular alignment over different IPDs are relatively large and incompatible with personal displays, which are required to be compact.
Many different methods have been devised for binocular alignment, however, so far, some proposed solutions require elaborate alignment methods, while others incorporate mechanical fixtures and alignment mechanisms, which significantly enlarge the personal display devices. The trade-off between the interest to reduce the size and weight of personal displays and the need for accurate binocular alignment is challenging and has resulted in many personal displays that are insufficiently aligned, and others that are inconveniently bulky.